Semiconductor devices are used in many electronic and other applications. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits.
One type of semiconductor device is a memory device, in which data is typically stored as a logical “1” or “0.” Memory devices may be static or dynamic. Dynamic memory devices need to be refreshed to “remember” the data, whereas static memory devices do not need to be refreshed to retain stored data.
One type of static memory device, also referred to in the art as a non-volatile memory (NVM) device, is a flash memory device. A flash memory device is an electrically erasable programmable read only memory (EEPROM) that is commonly used in computers, digital cameras, MP3 players, gaming systems, and memory sticks, as examples, although flash memory devices may be used in other applications, as well. Flash memory devices do not require power to retain stored data; they retain data even when the power source is disconnected. In flash memory devices, in-circuit wiring is used to erase predetermined sections or blocks of the chip by applying an electrical field to the entire chip, for example.
Flash memory devices typically comprise an array of flash memory cells. Flash memory cells are accessible for programming and retrieving data by an array of wordlines and bitlines coupled to the array of flash memory cells. Each flash memory cell comprises a floating gate and a control gate, which are separated by a thin insulator. Flash memory cells are programmed by applying a voltage to the control gate. Flash memory cells store a charge in the floating gate and are programmed using Fowler-Nordheim tunneling or channel hot electron injection from the channel or source and drain regions.
Recent flash memory applications include “embedded flash memory” and system on a chip (SoC) devices, in which an array of flash memory cells and peripheral circuitry for the flash memory cells are formed on a single chip or integrated circuit. The peripheral circuitry may comprise high voltage circuits, logic circuits for microcontrollers or processors, and other types of devices that may comprise transistors, diodes, bandgap devices, capacitors, inductors, and linear devices, as examples, although other types of devices may be included in the peripheral circuitry.
Embedded flash memory devices and SoC devices are more difficult to manufacture and require more manufacturing process steps and lithography masks to produce them.
What are needed in the art are improved methods of manufacturing embedded flash memory devices and structures thereof that require fewer dedicated lithography masks and processing steps for the flash memory portion of the chip.
Furthermore, flash memory is a relatively new technology, and there are limitations in further reducing the size of flash memory cells in current designs in the industry. Flash memory is more expensive to manufacture than traditional forms of memory, such as DRAMs.
Thus, what are also needed in the art are improved flash memory designs having lower manufacturing costs and reduced size.
Silicon-on-insulator (SOI) substrates have been used in the semiconductor industry in devices other than flash memory devices to achieve high performance and lower power dissipation for SoC applications. As an example, PowerPC™ microprocessors by International Business Machines (IBM) Corporation are manufactured on SOI substrates. However, SOI substrates have not yet been successfully used in flash memory devices because of a floating body effect that deleteriously affects the performance and reliability of the flash memory cells, to be described further herein.
Thus, what is also needed in the art is a means of integrating flash memory devices on SOI substrates for SoC devices, in order to achieve lower dissipation power and increased performance.